Numerous people have publicly claimed that there are serious issues with the number of adjacencies that may arise as a result of per-VLAN instances of any link-state routing protocol (such as IS-IS or OSPF). Yet per-VLAN instancing is—by far—the simplest and most elegant approach to using a link state routing protocol for determining paths to be used in shortest path bridging.
Approaches discussed thus far—that may be used to deal with this—are all essentially based on either limiting the scale to which shortest path bridging might extend in conjunction with VLANs, or complicating the link state routing interactions by introducing an un-natural divergence between the real VLAN topology and the effective forwarding paths to be applied.
These concerns stem from a belief that as many as 4K VLANs may be configured with a large overlap among edge bridges in VLAN membership, forcing as many as 16M adjacencies to be potentially maintained at each edge bridge.